The present invention relates to a cathode-ray tube in general and, in particular, to the structure of a glass panel section of a cathode-ray tube.
In a conventional cathode-ray tube, (as shown in an exemplary embodiment by 10 in FIG. 1) a phosphor screen is formed on the inner surface of a substantially rectangular shaped faceplate 22 of a glass panel section 20. A funnel section 40 having a deflection yoke device (not shown) therearound is sealed to a skirt 24 of the glass panel section 20 through a connective portion 30. A neck 50 extends from the funnel section 40, and an electron gun (not shown) for emitting an electron beam or electron beams is disposed within the neck 50. The envelope of the conventional cathode-ray tube 10 is comprised of the glass panel section 20, the funnel section 40 and the neck 50. The interior of the envelope is evacuated to a high vacuum (i.e., low pressure).
In the conventional cathode-ray tube of the type described above, the electron beam from the electron gun is deflected in accordance with, for example, the NTSC system. In a color cathode-ray tube, the electron beams are projected on the phosphor screen through a plurality of apertures of a shadow mask opposing the inner surface of the faceplate 22. To decrease difference between the length of an electron beam path from the electron gun to a peripheral portion of the phosphor screen (i.e., the peripheral region of the inner surface of the faceplate 22) and an electron beam path from the electron gun to the central region of the phosphor screen (i.e., the central region of the inner surface of the faceplate 22), and between deflection of the electron beam from the electron gun to the peripheral region of the phosphor screen and deflection of the electron beam from the electron gun to the central region thereof, the inner and outer surfaces of the conventional rectangular faceplate 22 are curved outward with given radii of curvature. For example, as shown in FIGS. 2A to 2C, a longitudinal axis (X--X) shown in FIG. 1 is normal to the tube axis and parallel to a line passing through center points of short sides of the the faceplate 22, a lateral axis (Y--Y) shown in FIG. 1 is normal to the tube axis and parallel to a line passing through center points of the long sides of the faceplate 22 and a diagonal axis (D--D) shown in FIG. 1 is normal to the tube axis and parallel to a line passing through the diagonal coners of the faceplate 22. If the inner surface radii of curvature along the lateral axis (Y--Y), the longitudinal axis (X--X) and the diagonal axis (D--D) of an inner surface 26 of the faceplate 22 are represented by Rsi, Rli and Rdi, respectively, and the outer surface radii of curvature along the lateral, longitudinal and diagonal axes of the outer surface thereof are Rso, Rlo and Rdo, respectively, the conventional faceplate 22 is generally designed and manufactured in a manner such that Rsi=Rli=Rdi=Ri and Rso=Rlo=Rdo=Ro, wherein Ri and Ro are predetermined values.
As shown in FIGS. 2A to 2C, when Hs, Hl and Hd denote respectively the various lengths of the skirt 24 at different portions thereof including the center portion of the short side, the center portion of the long side and the corner along the tube axis, respectively, the lengths of skirt 24 satisfy the inequality Hl&gt;Hs&gt;Hd whenever the outer surface radii of curvature are established in accordance with the foregoing relations. As is also apparent from FIGS. 2A to 2C, when ts, tl and td represent thicknesses of the faceplate 22 in the vicinity of the center portion of the short side, the center portion of the long side and the corner thereof, respectively, the thicknesses satisfy the inequality tl&lt;ts&lt;td in accordance with the relationships of distances between the tube axis and the center portion of the long side, between the tube axis and the center portion of the short side and between the tube axis and the corner, when the values Ri and Ro of the inner and outer surfaces radii of curvature are given as predetermined values, respectively, and the value Ri of the inner surface radii of curvature is equal to or smaller than the value Ro of the outer surface radii of curvature. In particular, in a large tube having a large distance between the tube axis and the peripheral portion of the faceplate 22, the difference between the thickness td (at the corner) and the thickness tl (at the center portion of the long side) or the difference between the thickness td (at the corner) and the thickness ts (at the center portion of the short side) is increased when screen size is increased.
In the glass panel section 20 of this type, any stress is focused to act on mechanically weak portions of the cathode-ray tube, and implosion tends to occur due to this stress. One of the mechanically weak portions is the connective portion 30 between the glass panel section 20 and the funnel section 40. In general accidental impact to the outer surface 28 of the faceplate 22 is transmitted to the connective portion 30 through skirt 24. Impact acting on the corner where the length of the skirt 24 is shortest remains substantially undamped, and such impact is therefore directly applied to the connective portion 30. An envelope having such a conventional glass panel section tends to be vulnerable to implosion. Other mechanically weak portions include the center portion of the long side, at which the difference between the inner pressure of the envelope and atmospheric pressure exists. Since the thicknesses satisfy the inequality tl&lt;ts&lt;td, the thickness tl at the center portion of the long side is smaller than that at any other peripheral portion, and this portion is accordingly vulnerable to implosion.